--- a/dom/events/IMEContentObserver.cpp
+++ b/dom/events/IMEContentObserver.cpp
@@ -346,16 +346,18 @@ IMEContentObserver::InitWithEditor(nsPre
     return false;
   }
 
   mDocShell = aPresContext->GetDocShell();
   if (NS_WARN_IF(!mDocShell)) {
     return false;
   }
 
+  MOZ_ASSERT(!WasInitializedWithPlugin());
+
   return true;
 }
 
 bool
 IMEContentObserver::InitWithPlugin(nsPresContext* aPresContext,
                                    nsIContent* aContent)
 {
   if (NS_WARN_IF(!aContent) ||
@@ -381,19 +383,27 @@ IMEContentObserver::InitWithPlugin(nsPre
   mEditableNode = aContent;
   mRootContent = aContent;
 
   mDocShell = aPresContext->GetDocShell();
   if (NS_WARN_IF(!mDocShell)) {
     return false;
   }
 
+  MOZ_ASSERT(WasInitializedWithPlugin());
+
   return true;
 }
 
+bool
+IMEContentObserver::WasInitializedWithPlugin() const
+{
+  return mDocShell && !mEditor;
+}
+
 void
 IMEContentObserver::Clear()
 {
   mEditor = nullptr;
   mSelection = nullptr;
   mEditableNode = nullptr;
   mRootContent = nullptr;
   mDocShell = nullptr;
@@ -419,18 +429,20 @@ IMEContentObserver::ObserveEditableNode(
   }
 
   mIsObserving = true;
   if (mEditor) {
     mEditor->AddEditorObserver(this);
   }
 
   mUpdatePreference = mWidget->GetIMEUpdatePreference();
-  if (mUpdatePreference.WantSelectionChange()) {
-    // add selection change listener
+  if (!WasInitializedWithPlugin()) {
+    // Add selection change listener only when this starts to observe
+    // non-plugin content since we cannot detect selection changes in
+    // plugins.
     nsCOMPtr<nsISelectionPrivate> selPrivate(do_QueryInterface(mSelection));
     NS_ENSURE_TRUE_VOID(selPrivate);
     nsresult rv = selPrivate->AddSelectionListener(this);
     NS_ENSURE_SUCCESS_VOID(rv);
   }
 
   if (mUpdatePreference.WantTextChange()) {
     // add text change observer
@@ -489,17 +501,17 @@ IMEContentObserver::UnregisterObservers(
     return;
   }
   mIsObserving = false;
 
   if (mEditor) {
     mEditor->RemoveEditorObserver(this);
   }
 
-  if (mUpdatePreference.WantSelectionChange() && mSelection) {
+  if (mSelection) {
     nsCOMPtr<nsISelectionPrivate> selPrivate(do_QueryInterface(mSelection));
     if (selPrivate) {
       selPrivate->RemoveSelectionListener(this);
     }
     mSelectionData.Clear();
     mFocusedWidget = nullptr;
   }
 
@@ -1280,17 +1292,17 @@ IMEContentObserver::MaybeNotifyCompositi
   FlushMergeableNotifications();
 }
 
 bool
 IMEContentObserver::UpdateSelectionCache()
 {
   MOZ_ASSERT(IsSafeToNotifyIME());
 
-  if (!mUpdatePreference.WantSelectionChange()) {
+  if (WasInitializedWithPlugin()) {
     return false;
   }
 
   mSelectionData.ClearSelectionData();
 
   // XXX Cannot we cache some information for reducing the cost to compute
   //     selection offset and writing mode?
   WidgetQueryContentEvent selection(true, eQuerySelectedText, mWidget);

--- a/dom/events/IMEContentObserver.h
+++ b/dom/events/IMEContentObserver.h
@@ -89,16 +89,17 @@ public:
    *                    Otherwise, false.
    */
   bool MaybeReinitialize(nsIWidget* aWidget,
                          nsPresContext* aPresContext,
                          nsIContent* aContent,
                          nsIEditor* aEditor);
   bool IsManaging(nsPresContext* aPresContext, nsIContent* aContent) const;
   bool IsManaging(const TextComposition* aTextComposition) const;
+  bool WasInitializedWithPlugin() const;
   bool IsEditorHandlingEventForComposition() const;
   bool KeepAliveDuringDeactive() const
   {
     return mUpdatePreference.WantDuringDeactive();
   }
   nsIWidget* GetWidget() const { return mWidget; }
   nsIEditor* GetEditor() const { return mEditor; }
   void SuppressNotifyingIME();
@@ -181,18 +182,17 @@ private:
            mNeedsToNotifyIMEOfPositionChange ||
            mNeedsToNotifyIMEOfCompositionEventHandled;
   }
 
   /**
    * UpdateSelectionCache() updates mSelectionData with the latest selection.
    * This should be called only when IsSafeToNotifyIME() returns true.
    *
-   * Note that this does nothing if mUpdatePreference.WantSelectionChange()
-   * returns false.
+   * Note that this does nothing if WasInitializedWithPlugin() returns true.
    */
   bool UpdateSelectionCache();
 
   nsCOMPtr<nsIWidget> mWidget;
   // mFocusedWidget has the editor observed by the instance.  E.g., if the
   // focused editor is in XUL panel, this should be the widget of the panel.
   // On the other hand, mWidget is its parent which handles IME.
   nsCOMPtr<nsIWidget> mFocusedWidget;

--- a/widget/IMEData.h
+++ b/widget/IMEData.h
@@ -34,17 +34,16 @@ class WritingMode;
  */
 struct nsIMEUpdatePreference final
 {
   typedef uint8_t Notifications;
 
   enum : Notifications
   {
     NOTIFY_NOTHING                       = 0,
-    NOTIFY_SELECTION_CHANGE              = 1 << 0,
     NOTIFY_TEXT_CHANGE                   = 1 << 1,
     NOTIFY_POSITION_CHANGE               = 1 << 2,
     // NOTIFY_MOUSE_BUTTON_EVENT_ON_CHAR is used when mouse button is pressed
     // or released on a character in the focused editor.  The notification is
     // notified to IME as a mouse event.  If it's consumed by IME, NotifyIME()
     // returns NS_SUCCESS_EVENT_CONSUMED.  Otherwise, it returns NS_OK if it's
     // handled without any error.
     NOTIFY_MOUSE_BUTTON_EVENT_ON_CHAR    = 1 << 3,
@@ -76,34 +75,29 @@ struct nsIMEUpdatePreference final
     return nsIMEUpdatePreference(aOther.mWantUpdates | mWantUpdates);
   }
 
   void DontNotifyChangesCausedByComposition()
   {
     mWantUpdates &= ~DEFAULT_CONDITIONS_OF_NOTIFYING_CHANGES;
   }
 
-  bool WantSelectionChange() const
-  {
-    return !!(mWantUpdates & NOTIFY_SELECTION_CHANGE);
-  }
-
   bool WantTextChange() const
   {
     return !!(mWantUpdates & NOTIFY_TEXT_CHANGE);
   }
 
   bool WantPositionChanged() const
   {
     return !!(mWantUpdates & NOTIFY_POSITION_CHANGE);
   }
 
   bool WantChanges() const
   {
-    return WantSelectionChange() || WantTextChange();
+    return WantTextChange();
   }
 
   bool WantMouseButtonEventOnChar() const
   {
     return !!(mWantUpdates & NOTIFY_MOUSE_BUTTON_EVENT_ON_CHAR);
   }
 
   bool WantChangesCausedByComposition() const

--- a/widget/PuppetWidget.cpp
+++ b/widget/PuppetWidget.cpp
@@ -816,17 +816,16 @@ PuppetWidget::GetIMEUpdatePreference()
   if (mInputContext.mIMEState.mEnabled == IMEState::PLUGIN) {
     // But if a plugin has focus, we cannot receive text nor selection change
     // in the plugin.  Therefore, PuppetWidget needs to receive only position
     // change event for updating the editor rect cache.
     return nsIMEUpdatePreference(mIMEPreferenceOfParent.mWantUpdates |
                                  nsIMEUpdatePreference::NOTIFY_POSITION_CHANGE);
   }
   return nsIMEUpdatePreference(mIMEPreferenceOfParent.mWantUpdates |
-                               nsIMEUpdatePreference::NOTIFY_SELECTION_CHANGE |
                                nsIMEUpdatePreference::NOTIFY_TEXT_CHANGE |
                                nsIMEUpdatePreference::NOTIFY_POSITION_CHANGE );
 #else
   // B2G doesn't handle IME as widget-level.
   return nsIMEUpdatePreference();
 #endif
 }
 
@@ -892,19 +891,18 @@ PuppetWidget::NotifyIMEOfSelectionChange
   // Therefore, we don't need to query text content again here.
   mContentCache.SetSelection(
     this, 
     aIMENotification.mSelectionChangeData.mOffset,
     aIMENotification.mSelectionChangeData.Length(),
     aIMENotification.mSelectionChangeData.mReversed,
     aIMENotification.mSelectionChangeData.GetWritingMode());
 
-  if (mIMEPreferenceOfParent.WantSelectionChange() &&
-      (mIMEPreferenceOfParent.WantChangesCausedByComposition() ||
-       !aIMENotification.mSelectionChangeData.mCausedByComposition)) {
+  if (mIMEPreferenceOfParent.WantChangesCausedByComposition() ||
+      !aIMENotification.mSelectionChangeData.mCausedByComposition) {
     mTabChild->SendNotifyIMESelection(mContentCache, aIMENotification);
   } else {
     mTabChild->SendUpdateContentCache(mContentCache);
   }
   return NS_OK;
 }
 
 nsresult

--- a/widget/android/nsWindow.cpp
+++ b/widget/android/nsWindow.cpp
@@ -3364,19 +3364,17 @@ nsWindow::GetInputContext()
 nsIMEUpdatePreference
 nsWindow::GetIMEUpdatePreference()
 {
     // While a plugin has focus, nsWindow for Android doesn't need any
     // notifications.
     if (GetInputContext().mIMEState.mEnabled == IMEState::PLUGIN) {
       return nsIMEUpdatePreference();
     }
-    return nsIMEUpdatePreference(
-        nsIMEUpdatePreference::NOTIFY_SELECTION_CHANGE |
-        nsIMEUpdatePreference::NOTIFY_TEXT_CHANGE);
+    return nsIMEUpdatePreference(nsIMEUpdatePreference::NOTIFY_TEXT_CHANGE);
 }
 
 nsresult
 nsWindow::SynthesizeNativeTouchPoint(uint32_t aPointerId,
                                      TouchPointerState aPointerState,
                                      LayoutDeviceIntPoint aPoint,
                                      double aPointerPressure,
                                      uint32_t aPointerOrientation,

--- a/widget/cocoa/nsChildView.mm
+++ b/widget/cocoa/nsChildView.mm
@@ -1929,21 +1929,20 @@ nsChildView::ExecuteNativeKeyBinding(Nat
 
   NativeKeyBindings* keyBindings = NativeKeyBindings::GetInstance(aType);
   return keyBindings->Execute(aEvent, aCallback, aCallbackData);
 }
 
 nsIMEUpdatePreference
 nsChildView::GetIMEUpdatePreference()
 {
-  // While a plugin has focus, IMEInputHandler doesn't need any notifications.
-  if (mInputContext.mIMEState.mEnabled == IMEState::PLUGIN) {
-    return nsIMEUpdatePreference();
-  }
-  return nsIMEUpdatePreference(nsIMEUpdatePreference::NOTIFY_SELECTION_CHANGE);
+  // XXX Shouldn't we move floating window which shows composition string
+  //     when plugin has focus and its parent is scrolled or the window is
+  //     moved?
+  return nsIMEUpdatePreference();
 }
 
 NSView<mozView>* nsChildView::GetEditorView()
 {
   NSView<mozView>* editorView = mView;
   // We need to get editor's view. E.g., when the focus is in the bookmark
   // dialog, the view is <panel> element of the dialog.  At this time, the key
   // events are processed the parent window's view that has native focus.

--- a/widget/gtk/IMContextWrapper.cpp
+++ b/widget/gtk/IMContextWrapper.cpp
@@ -328,17 +328,17 @@ IMContextWrapper::GetIMEUpdatePreference
 {
     // While a plugin has focus, IMContextWrapper doesn't need any
     // notifications.
     if (mInputContext.mIMEState.mEnabled == IMEState::PLUGIN) {
       return nsIMEUpdatePreference();
     }
 
     nsIMEUpdatePreference::Notifications notifications =
-        nsIMEUpdatePreference::NOTIFY_SELECTION_CHANGE;
+        nsIMEUpdatePreference::DEFAULT_CONDITIONS_OF_NOTIFYING_CHANGES;
     // If it's not enabled, we don't need position change notification.
     if (IsEnabled()) {
         notifications |= nsIMEUpdatePreference::NOTIFY_POSITION_CHANGE;
     }
     nsIMEUpdatePreference updatePreference(notifications);
     return updatePreference;
 }
 

--- a/widget/windows/IMMHandler.cpp
+++ b/widget/windows/IMMHandler.cpp
@@ -390,17 +390,16 @@ IMMHandler::GetKeyboardCodePage()
 }
 
 // static
 nsIMEUpdatePreference
 IMMHandler::GetIMEUpdatePreference()
 {
   return nsIMEUpdatePreference(
     nsIMEUpdatePreference::NOTIFY_POSITION_CHANGE |
-    nsIMEUpdatePreference::NOTIFY_SELECTION_CHANGE |
     nsIMEUpdatePreference::NOTIFY_MOUSE_BUTTON_EVENT_ON_CHAR);
 }
 
 // used for checking the lParam of WM_IME_COMPOSITION
 #define IS_COMPOSING_LPARAM(lParam) \
   ((lParam) & (GCS_COMPSTR | GCS_COMPATTR | GCS_COMPCLAUSE | GCS_CURSORPOS))
 #define IS_COMMITTING_LPARAM(lParam) ((lParam) & GCS_RESULTSTR)
 // Some IMEs (e.g., the standard IME for Korean) don't have caret position,

--- a/widget/windows/TSFTextStore.cpp
+++ b/widget/windows/TSFTextStore.cpp
@@ -4541,17 +4541,16 @@ TSFTextStore::CreateAndSetFocus(nsWindow
 nsIMEUpdatePreference
 TSFTextStore::GetIMEUpdatePreference()
 {
   if (sThreadMgr && sEnabledTextStore && sEnabledTextStore->mDocumentMgr) {
     RefPtr<ITfDocumentMgr> docMgr;
     sThreadMgr->GetFocus(getter_AddRefs(docMgr));
     if (docMgr == sEnabledTextStore->mDocumentMgr) {
       nsIMEUpdatePreference updatePreference(
-        nsIMEUpdatePreference::NOTIFY_SELECTION_CHANGE |
         nsIMEUpdatePreference::NOTIFY_TEXT_CHANGE |
         nsIMEUpdatePreference::NOTIFY_POSITION_CHANGE |
         nsIMEUpdatePreference::NOTIFY_MOUSE_BUTTON_EVENT_ON_CHAR |
         nsIMEUpdatePreference::NOTIFY_DURING_DEACTIVE);
       // TSFTextStore shouldn't notify TSF of selection change and text change
       // which are caused by composition.
       updatePreference.DontNotifyChangesCausedByComposition();
       return updatePreference;